The present invention relates to a waveguide with non-inclined radiating slots of the type comprising slots perpendicular to the axis of the waveguide, cut out on a narrow wall of the waveguide with a spacing that is substantially but not precisely equal to a half wavelength of operation in the waveguide, and means for the excitation of each of the slots.
Slotted waveguides are often used as linear arrays of radiating sources in array antennas, for example in radar. Their advantages are low cost and low losses. To obtain radiation close to the normal to the waveguide, as well as efficient matching, it is necessary to have, firstly, a distance between successive slots that is close to .lambda. g/2 where .lambda. g is the wavelength in the waveguide and, secondly, an additional phase shift of .pi. between two consecutive slots.
These conditions can be fulfilled with slots positioned on the broad wall of a rectangular-sectioned waveguide, or on its narrow wall. The arrangement with slots on the broad wall has several drawbacks, notably a substantial pitch between successive waveguides, which limits the scanning angle of the beam in a plane perpendicular to the waveguides. It is therefore preferred to use slots on the narrow wall of the waveguides.
If the slots are perpendicular to the axis of the waveguide, there is no energy coupling between the slots and the waveguide, and the radiation is zero.
In a first approach, therefore, the slots are inclined alternately on one side and then on the other, to obtain the necessary conditions specified here above. However, the drawback of this approach, resulting from the inclination of the slots, is that it entails the radiation of a crossed-polarization component which may reach levels incompatible with the efficient operation of the antenna using these waveguides.
Another known approach, therefore, consists in using slots that are not inclined (perpendicular to the axis of the waveguide) and exciting them by means of an obstacle positioned in the waveguide (iris, rods etc.).
In particular, the U.S. Pat. No. 4,435,715 (Hughes Aircraft) describes a waveguide with non-inclined slots in which the excitation of a slot is obtained by positioning conductive rods on either side of the slot. Each slot is positioned between an edge of the slot and one of the broad walls of the waveguide. However, the drawback of an approach such as this is that it is costly. Indeed, the rods have to be fixed individually inside the waveguide, for example by dip soldering. Furthermore, the use of rods entails major drawbacks as regards operations involving microwaves. For, this approach has a degree of frequency selectivity, the consequence of which is a restricted passband and a relatively unsatisfactory standing-wave ratio (SWR). Furthermore, the method of coupling by means of rods entails the use of relatively wide slots, resulting in a residual level of crossed polarization which may may raise problems for certain applications.